Methods and systems for the purpose at hand which have seen primary commercialization are of type involving passive article- attached devices, i.e., of type not including power sources. Such devices have, in prior patents, been termed receptor-reradiators and are adapted to reradiate or retransmit energy incident thereon in preselected relation thereto. By way of example, article tags have included radiator-receptor devices having an antenna, for receiving microwave and modulated low-frequency energy transmitted into and beyond a surveillance or control area, and a nonlinear element, such as a diode, directly connected to the antenna. The radiator-receptor output energy is thereby a reradiation of the microwave and modulated low-frequency incident energy.
As is well known, radiation pattern control, in relatively small areas, such as retail establishments, libraries, etc., is difficult in proportion to increase in the frequency level. The microwave energy is quite difficult to control in pattern. Thus, the industry has looked to the microwave as a carrier and to a lower, more pattern controllable, transmitted frequency radiated also into the area under coverage with a modulation characteristic, and to establishing alarm conditions based upon joint receipt and reradiation of the two diverse frequency incident energies. Alarm condition is sensed upon detection of the modulation characteristic of the low-frequency reradiated signal.
Despite the refined detection capability of the multiple frequency systems, and the pattern control aspect of the low-frequency transmission in the described system, a degree of ambiguity can still attend same, based on the variation from intended pattern of the relatively controllable low-frequency transmission. The desired, well-defined line of demarcation as between the surveillance zone and adjacent article display zones is at times not attained and this gives rise, on occasion, to incorrect alarm indication in intended display areas and corresponding diminution of the usable display area.
Plural low-frequency modulated signals have been considered, and a quite recent development has looked to the selective de-sensitizing of a receiver as respects signals reradiated from the article display area by use of unmodulated auxiliary low-frequency transmissions, for defining the demarcation line and enhancing display area usage.
Based on the possible ambiguities in the foregoing practices and systems, the industry continues to look to alternative approaches to the above, including use of independently-powered article affixed devices having safety-pins or like mechanically responsive alarming apparatus, as in commonly-assigned Nathans U.S. Pat. No. 3,713,133, issued on Jan. 23, 1973 and U.S. patent application, Ser. No. 474,966, filed on Mar. 4, 1983 in the names of Robert C. Boyd and Charles L. LaPatra, entitled "Electronic Article Surveillance Security System." In the system of the '966 application, the article-attachable tag unit (alarm device) has a switch, which isooperated upon efforts to remove the tag unit from the article, and an audible tone generator which is activated upon such switch operation. Further, the tag unit has a receiving antenna and a related received frequency detector, which will also operate the alarm device. A transmitter radiates energy at the receiver detectable frequency into a control area for surveillance thereof.
While the system and device of the '966 application are thought to be viable as an alternative to the systems previously above discussed, in providing powered circuitry at the tag itself both for discernment of a specific frequency and of removal of the tag to generate an alarm localized to the immediate tag location, such system and device are in rudimentary character in the '966 application. For instance, the alarm device is disarmable only upon mechanical interfitting thereof with a "negator", typically fixedly located, e.g., at a checkout location. Further, such system contemplates only unidirectional transmission, i.e., to the tag alarm device. In brief, capabilities of usage of the '966 system are limited by reason of the simplistic character of the device therein.